Device for triggering a lighting device in a motor vehicle

ABSTRACT

A device for triggering a lighting device of a motor vehicle has a control arrangement ( 10 ) which generates at least one pulse width modulated control signal ( 16 ) for at least one switching means ( 18 ) via which at least one lighting device ( 20 ) may be supplied with electric power. The control arrangement ( 10 ) receives a measure of the power supply voltage (UB) supplied at least to the lighting device ( 20 ) in order to influence the control signal ( 16 ) as a function of the power supply voltage (UB). The control arrangement ( 10 ) determines the control signal ( 16 ) as a function of at least one additional operating parameter (v)—in addition to the power supply voltage (UB)—of the motor vehicle.

BACKGROUND INFORMATION

[0001] The present invention is directed to a device for triggering alighting device in a motor vehicle according to the preamble of theindependent claim. European Patent Application 935 404 has described acircuit for stabilizing the voltage for a lamp, in particular forheadlights on a motor vehicle. The lamp voltage is stabilized at areduced power loss by the fact that an additional time-delay switchcontrollable by a trigger circuit is included in the lamp circuit; whenthe power supply voltage is lower than the predefined nominal operatingvoltage of the lamp, the trigger circuit keeps the time-delay switchswitched on continuously; and at a higher power supply voltage than thenominal operating voltage of the lamp, the trigger circuit periodicallyswitches the time-delay switch on and off, the ratio of the on time tothe duration of the period corresponding to the square of the ratio ofthe nominal operating voltage to the applied power supply voltage.

[0002] The object of the present invention is to further stabilize thelighting intensity of a lighting device situated in a motor vehicle,even for different operating states of the vehicle. This object isachieved through the features of the independent claim.

ADVANTAGES OF THE INVENTION

[0003] The device according to the present invention for triggering alighting device of a motor vehicle has a control arrangement whichgenerates at least one pulse width modulated control signal for at leastone switching means via which at least one lighting device may besupplied with electric power. The control arrangement receives a measureof the power supply voltage which supplies power at least to thelighting device for influencing the control signal as a function of thepower supply voltage. The control arrangement determines the controlsignal as a function of at least one additional operating parameter ofthe vehicle in addition to the power supply voltage. Thanks to thevariable adaptation of the pulse width modulated control signal,different operating situations of the motor vehicle may be taken intoaccount in which, as experience has shown, light intensities whichfluctuate to different extents may occur. The driving speed is availableas one possible operating parameter. The battery is usually under thegreatest load when the vehicle is standing still, so that stabilizationhas priority. However, the brightness of the headlights need not bemaximized when the vehicle is stationary. Driving speed as a possibleoperating parameter is taken into account in determining the controlsignal. Furthermore, flickering of the lights caused by other electricloads being turned on or off is hardly noticeable while driving. Thus,at high speeds, the stabilization function is not so strongly emphasizedas adequate brightness of the lamp, i.e., the headlights.

[0004] In an expedient refinement, the control arrangement specifies amaximum pulse width as a control signal when the power supply voltagedrops below a lower limit. The lower limit is variable according to thepresent invention as a function of at least one operating parameter ofthe vehicle. The calculation procedure for determining the pulse widthis stored in the control arrangement as a function of the lower limitvalue. Only this one value need be varied as a function of the operatingparameter in order to take into account the prevailing operatingsituation of the vehicle in setting the control signal. Thecorresponding algorithm thus has a relatively simply structure.

[0005] In an expedient refinement, the control signal may depend on thecharge state of the automotive battery. The charge state is determined,for example, in conjunction with the power supply voltage beforestarting the vehicle and is used for setting the new lower limit value,for example. In the case of a high power supply voltage, indicating afully charged automotive battery, the lower limit value may be sethigher, because stabilization then plays a subordinate role. When thebattery is relatively low, however, the tendency of the lighting deviceto flicker is much greater. Therefore, fluctuations in power supplyvoltage in a larger stabilization range are taken into account insetting the control signal.

[0006] In an expedient refinement, the control arrangement specifies aminimum pulse width as the control signal when the power supply voltageexceeds an upper limit value. The minimum pulse width is variable as afunction of the operating parameter. Due to this definition of theminimum pulse width, the stabilization range of the control arrangementmay also be adapted to the prevailing operating situation. In particularwith an increase in driving speed, the minimum pulse width is increasedfor adaptation of the stabilization range. In addition, this providesovervoltage protection and lengthens the lifetime of the battery.

[0007] Additional expedient refinements are derived from additionaldependent claims and from the description.

DRAWING

[0008] One exemplary embodiment of the device according to the presentinvention is illustrated in the drawing and is described in greaterdetail below.

[0009]FIG. 1 shows a block diagram and FIG. 2 shows the triggering for alighting device as a function of operating parameters.

[0010] An output signal of a first operating element 12 and an outputsignal of a second operating element 14 are sent to a microcontroller10. Microcontroller 10 generates a trigger signal 16 according to acertain computation algorithm, this signal then triggering a switchingmeans 18, opening or closing it. A lamp 20 is supplied with power supplyvoltage UB provided by a battery 22 via switching means 18. Power supplyvoltage UB is also sent to microcontroller 10, which also exchanges dataover a bus system 24, over which driving speed v detected by a speedsensor 26 is sent to microcontroller 10.

[0011] Trigger signal 16 is a pulse width modulated signal which variesbetween a maximum pulse width PWM_(max) of 100% and a minimum minPWM_(min) of minimum pulse width PWM_(min). FIG. 2 shows a linearrelationship between pulse width PWM and power supply voltage UB. Threecharacteristic curves are shown as a function of driving speed v as anexample. Lower limit value U_(min) decreases from max U_(min) to minU_(min) with a decrease in driving speed. When power supply voltage UBreaches or falls below lower limit value U_(min), microcontroller 10generates a control signal 16 having a maximum pulse width PWM_(max) of100%, for example, i.e., switching means 18 the assumes a continuous onstate. In the range between lower limit value U_(min) and upper limitvalue U_(max), pulse width PWM decreases linearly with an increase inoperating voltage. If operating voltage UB reaches or exceeds upperlimit value U_(max), pulse width PWM remains constantly at the level ofminimum pulse width PWM_(min). Upper limit value U_(max) retains itsvalue independently of a change in driving speed v. However, minimumpulse width PWM_(min) increases with an increase in driving speed v. Itthus fluctuates between minimum min PWM_(min) of minimum pulse widthPWM_(min) and maximum max PWM_(min) of minimum pulse width PWM_(min).With an increase in driving speed v, there is a decrease in the linear,directly proportional range within which the varying operating voltageUB is stabilized by adjusting pulse width PWM of control signal 16 as afunction of operating voltage UB. Microcontroller 10 generates pulsewidth modulated trigger signal 16 with which switching means 18 isswitched on or off. Pulse width is understood to refer to the durationof the on time based on a fixed cycle time. The brightness of lamp 20may vary due to variations in pulse width PWM. Microcontroller 10 shouldthen ensure that a uniform light intensity is always achieved, even whenthere is a fluctuation in power supply voltage UB. Flickering of lamp 20in particular is to be prevented, in particular when there are greatload changes which may directly affect power supply voltage UB. To thisend, microcontroller 10 detectors power supply voltage UB and determinesthe instantaneous pulse width according to the following algorithm:

PWM=(((U _(max) −UB)[÷](U _(max) −U _(min)))·(PWM _(max) −PWM_(min)))+PWM _(min)

[0012] where U_(max)=upper limit value

[0013] UB=power supply voltage

[0014] [U_(min)]=lower limit value

[0015] [PWM_(max)]=maximum pulse width

[0016] [PWM_(min)]=minimum pulse width

[0017] This yields the linear relationship for the average operatingvoltage range (U_(min)<UB<U_(max)) as illustrated in FIG. 2. If theabsolute value of power supply voltage UB detected is below lower limitvalue U_(min), then switching means 18 is triggered by trigger signal 16having maximum pulse width PWM_(max). This may amount to 100%, forexample, so that in this case switching means 18 would be closedcontinuously. Maximum possible power supply voltage UB is thus availableto lamp 20 to an unlimited extent. Lamp 20 is thus triggered at themaximum possible power and therefore at the maximum possible intensity,in particular in cases of heavy load (UB<U_(min)).

[0018] If power supply voltage UB is between lower limit value U_(min)and upper limit value U_(max) (U_(min)<U_(max)), then pulse width PWM isdirectly proportional to UB. When power supply voltage UB reaches upperlimit value U_(max) (UB<U_(max)), lamp 20 is triggered at a maximumpulse width PWM_(min) to ensure adequate lighting in any case.

[0019]FIG. 2 shows as an example a linear relationship between powersupply voltage UB and pulse width PWM. However, to keep the lightintensity constant as a function of the power supply voltage, aparabolic or other mathematical relationship between pulse width PWM andpower supply voltage UB could also be used. The characteristic curveaccording to FIG. 2 is to be adapted to the particular type of lightingdevice accordingly.

[0020] In agreement with FIG. 2, the characteristic line is now shiftedas a function of the operating parameter to adapt the stabilization ofthe light intensity to different operating states. In the exemplaryembodiment according to FIG. 2, driving speed v is used as the operatingparameter. This speed is detected by a speed sensor 26 and is conveyedto microcontroller 10 via bus system 24, for example. Lower limit valueU_(min) varies between a minimum lower limit value min U_(min) and amaximum lower limit value max U_(min) as a function of the drivingspeed. With an increase in driving speed v, the absolute value of lowerlimit value U_(min) increases. The characteristic curves illustrated inFIG. 2 ensure that a great measure of stabilization is achieved when thevehicle is at a standstill (v=0). Therefore, the stabilization functionintervenes earlier in the form of the linear characteristic curve (inthe range of U_(min)<UB<U_(max)) than when the vehicle is moving. If thevehicle is moving (v>0), then the maximum brightness of lamp 20 inparticular is of interest for the user. Therefore, pulse width PWMremains at maximum value PWM_(max) for a greater operating voltage rangethan at low driving speeds. The stabilization effect is thus reducedwith an increase in speed in favor of maximum brightness.

[0021] With an increase in driving speed v, minimum pulse widthPWM_(min) is increased while upper limit value U_(max) remains constant.

[0022] The speed dependence of lower limit value U_(min) and/or minimumpulse width PWM_(min) is stored in microprocessor 10 in the form oftables or as a direct mathematical relationship. For calculation ofinstantaneous pulse width PWM, lower limit value U_(min), adaptedaccording to driving speed v, and minimum pulse width PWM_(min) are usedas the basis for the computation algorithm.

[0023] In an alternative embodiment (not shown here), upper limit valueU_(max) could also be designed to be dependent on the operatingparameter and to decrease in absolute value with an increase in drivingspeed v.

[0024] In an alternative embodiment, the charge state of battery 22 isused as an operating: parameter. Before starting the vehicle,microcontroller 10 detects power supply voltage UB as a measure of thecharge state of battery 22. The degree of stabilization may be increasedat a lower charge state of battery 22 by increasing the operatingvoltage range by defining the pulse width as a function of operatingvoltage (U_(min)<UB<U_(max)) The situation of a low charge state ofbattery 22 thus corresponds to that of low driving speeds v. Inagreement with FIG. 2, there is similarly an adaptation of lower limitvalue U_(min) and minimum pulse width PWM_(min).

[0025] In addition, the output signals of operating elements 12, 14 aresent to the microcontroller. Lamp 20 is activated or deactivated as afunction of these signals, with pulse width PWM thus determined.

[0026] Lamp 20 may preferably be the headlight of a motor vehicle or theinterior lighting. However, the present invention is not limited tothese cases.

What is claimed is:
 1. A device for triggering a lighting device of amotor vehicle, having a control arrangement (10) which generates atleast one pulse width modulated control signal (16) for at least oneswitching means (18) via which at least one lighting device (20) may besupplied with electric power, the control arrangement (10) receiving ameasure of the power supply voltage (UB) supplying at least the lightingdevice (20) in order to influence the control signal (16) as a functionof the power supply voltage (UB), wherein the control arrangementdetermines the control signal (16) as a function of at least oneadditional operating parameter (v)—in addition to the power supplyvoltage (UB)—of the motor vehicle.
 2. The device as recited in claim 1,wherein the control arrangement (10) specifies a maximum pulse width(PWM_(max)) as the control signal (16) when the power supply voltage(UB) drops below a lower limit value (U_(min)).
 3. The device as recitedin one of the preceding claims, wherein the control arrangement (10)specifies a minimum pulse width (PWM_(min)) as the control signal (16)when the power supply voltage (UB) exceeds an upper limit value(U_(max)).
 4. The device as recited in one of the preceding claims,wherein the lower limit value (U_(min)) and/or the minimum pulse width(PWM_(min)) is variable as a function of the operating parameter (v). 5.The device as recited in one of the preceding claims, wherein a signalindicating whether the vehicle is moving or at a standstill is used asthe operating parameter (v).
 6. The device as recited in one of thepreceding claims, wherein the driving speed (v) and/or the charge stateof a battery (22) of the motor vehicle is used as the operatingparameter.
 7. The device as recited in one of the preceding claims,wherein the lower limit value (U_(min)) increases with an increase indriving speed (v).
 8. The device as recited in one of the precedingclaims, wherein the lower limit value (U_(min)) increases with anincrease in the charge state of the battery (22).
 9. The device asrecited in one of the preceding claims, wherein the power supply voltage(UB) is used as a measure of the charge state of the battery (22). 10.The device as recited in one of the preceding claims, wherein at leastone signal of an operating element (12, 14) is sent to the controlarrangement (10), and the control signal (16) is generated as a functionthereof.